mm/sl[aou]b: Extract a common function for kmem_cache_destroy

kmem_cache_destroy does basically the same in all allocators.

Extract common code which is easy since we already have common mutex
handling.

Reviewed-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

diff --cc mm/slab.c
Simple merge

diff --cc mm/slab.h
Simple merge

diff --cc mm/slab_common.c
Simple merge

diff --cc mm/slob.c
Simple merge

diff --cc mm/slub.c
Simple merge